Stability and precision over extended periods of time for a variety of different wavelengths are becoming increasingly important for various optical systems. For example, in next generation extra-solar planetary discovery and investigation, it is desirable to achieve a radial velocity (RV) measurement precision at level of 0.1 meters per second (m/s) annually (as compared to current precision levels in the range of approximately 1 m/s per month). In addition, the range of wavelengths in RV measurements is expanding to include the near-IR portion of the electromagnetic spectrum in addition to the visible portion. To this end, wavelength calibration tools are necessary to ensure appropriate control of the precision of the measurement system over time.
Light sources that use traditional gas emission or absorption lines, such as Thorium-Argon emission lamps or Iodine absorption cells, have been used as calibration standards in the visible light range. However, their lack of calibration sensitivity, variation due to aging, and/or temperature instability may be prohibitive in achieving high-precision in an optical system over an extended period of time. Moreover, these light sources emit visible light and thus may be unable to provide wavelength calibration outside of the visible portion of the electromagnetic spectrum.
High-precision laser combs have been used for wavelength calibration. While the laser comb may have a wide bandwidth and the ability to provide extended wavelength coverage through amplification and nonlinear conversion, they require a significant financial investment, at least initially. In addition, laser combs face many complex issues in terms of maintenance, operation, and implementation, especially with regard to calibration of RV measurement systems. For example, the wavelength peaks generated by the laser combs may be too closely spaced, thereby requiring an external filter, such as a high-finesse etalon or multi-etalon chain, to increase the spacing between wavelength peaks. Reliably matching the etalon cavity length to the laser cavity length can also pose a significant challenge. Moreover, the external filter may have an effect on the wavelength stability of the system in addition to limiting the bandwidth of the laser source.